The next generation of commercial high capacity Ka frequency band (i.e., 26.5-40 GHz) satellite-based broadband systems promise high data rates for the end user at a low cost. These systems typically use multiple gateway terminals to communicate with a large number of user terminals, with the satellite between the gateway terminal and the user terminals. A forward link of a satellite communications system may consist of forward uplink transmissions from a gateway terminal to a satellite, a “bent pipe” repeater at the satellite, and forward downlink transmissions to a group of user terminals located in a common spot beam. A return link of a satellite communications system may consist of return uplink transmissions from user terminals in a common spot beam to a satellite, and return downlink transmissions from the satellite to a gateway terminal servicing the spot beam.
One example of a satellite system is a Time Division Multiple Access (TDMA) system, where each user shares the same channel but uses different timeslots. A variation on a TDMA system is a Multi-Frequency TDMA (MF-TDMA) system, where each user could be made to hop across different frequency channels on a burst-by-burst basis. In some implementations of an MF-TDMA or a TDMA system, a return link scheduler at the gateway terminal can allocate different non-overlapping timeslots to different users.
With increasing customer demand for higher upload speeds on the return link, these satellite systems typically employ multiple channels, each at different symbol rates (e.g., 20 Msps, 10 Msps, 5 Msps, etc.) on the return link. For the same Forward Error Correction (FEC) encoding on all the return link channels, higher symbol rates result in higher end user speeds. Thus, for example, with quadrature phase-shift keying (QPSK) modulation and an FEC code rate of ½, a user of a 20 Msps return link channel would be able to achieve 20 Mbps upload speeds, while a user of a 5 Msps return link channel would only be able to achieve 5 Mbps upload speeds with the same modulation and code rate.
Thus, it is desirable from a user experience point of view to have the user terminals communicate to the gateway terminal on a return link channel at the highest possible symbol rate. The highest symbol rate at which a particular user terminal may be capable of operating can depend on the particular user terminal. This dependency might be due to a variety of factors, such as amplifier wattage, position in the spot beam, weather conditions, pointing accuracy, the class of service obtained, etc. Typically, if other factors (e.g., FEC and modulation) remain constant, higher symbol rates require higher transmit power to achieve the same error performance achieved with lower transmit power at lower symbol rates.
In conventional systems, an appropriate symbol rate is chosen for each user terminal in a non-adaptive, hard-coded fashion (e.g., hard-coded in a factory-installed configuration file in the user terminal) or by trial and error. Typically, there is no mechanism for adaptively determining the maximum symbol rate of the return link for a particular user terminal.